The ionosphere is created by ionization of the upper atmosphere by ultraviolet and X-rays, and is divided in to D, E and F layers.The small wavelength of microwaves allows them to be directed in narrow beams by aperture antennas such as parabolic dishes and horn antennas, so they are used for point-to-point communication and data links and for radar.This frequency range is use for most radar transmitters, wireless LANs, satellite communication, microwave radio relay links, and numerous short range terrestrial data links.Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz).Millimetre-length electromagnetic waves were first investigated in the 1890s by Bengali-Indian scientist Jagadish Chandra Bose.The short wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.Super high frequency (SHF) is the ITU designation for radio frequencies (RF) in the range between 3 and 30 gigahertz(GHz).Absorption by humidity in the atmosphere is significant except in desert environments, and attenuation by rain (rain fade) is a serious problem even over short distances.Since the ionization varies with altitude, signals are not reflected, but refracted by the ionosphere.4
Frequencies in the SHF range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band.Compared to lower bands, radio waves in this band have high atmospheric attenuation: they are absorbed by the gases in the atmosphere.However the short propagation range allows smaller frequency reuse distances than lower frequencies.